Conventionally, a laminated core to be used as a stator core of a stepping motor or the like is formed in such a structure that a plurality of core pieces 10 of magnetic material are laminated, similarly to that which is shown in FIGS. 1 and 2. Each core piece 10 is formed with such a flat shape as illustrated, by progressive forming by a press, in advance. Thereafter, one core piece 10 is put on the top of another core piece 10, and half blanking is performed in caulking portions 14 as illustrated so that the two core pieces 10 are fitted and fixed to each other. Such a process is repeated to fix a large number of core pieces 10 to each other sequentially to thereby form a laminated core 1.
The laminated core 1 is provided with an outer frame portion 11 formed in the outer circumferential portion, and teeth portions 18 formed in the inner circumferential portion. The teeth portions 18 are constituted by, for example, eight bridge portions 12 projecting inward from the inner circumferential surface of the outer frame portion 11, and magnetic pole portions 13 formed on the forward ends of these bridge portions 12, respectively. A surface-roughed shape constituted by mountain portions 13a and groove portions 13b formed alternately in the direction of rotation of a not-shown rotor is formed in the inner circumferential edge portion of each of the magnetic pole portions 13. The area surrounded by these eight magnetic pole portions 13 forms an inner hole 16 which is substantially circular, so that a not-shown rotor is received in this inner hole 16.
However, in a motor in which such a laminated core as mentioned above is used as a stator core, large noise may be caused when the motor is driven. Though this noise is considered to be caused for various reasons, the present inventors have found, through various analyses, that the main reason is in that the core pieces 10 of the stator core vibrate in the vicinities of the magnetic pole portions 13, and the core pieces 10 resonate in the case where a certain condition is satisfied.
In order to restrain such vibrations or resonance, it is considered necessary to fix the core pieces 10 to each other at a portion each of the teeth portions 18.
Therefore, caulking portions 15 was provided in the bridge portions 12 of the respective teeth portions by way of experiment. As a result, it was found that the above-mentioned vibrations or resonance could be reduced effectively. However, in this case, the number of the caulking portions 15 increased correspondingly to the number of the teeth portions 18, so that not only it was necessary to make mold adjustment which requires high technique and skill, but also the durability of the mold was required unreasonably. Accordingly, the manufacturing cost was increased.
Further, caulking portions were provided in the magnetic pole portions 13 of the teeth portions 18 by way of experiment. In this case, not only it was difficult to perform half blanking but also it was difficult to ensure the strength of the mold because the plane shapes of the respective core pieces 10 are formed to be narrow in the magnetic pole portions 13. Accordingly, manufacturing was difficult in practical use.
Furthermore, another method in which the core pieces 10 are bonded with each other by a bonding agent or tape with a bonding agent may be considered. However, not only it is comparatively difficult to automate this bonding process and it requires high cost to provide equipment therefor, but also there is such a danger that the bonding agent is eluted in the process of cleansing the stator core to make the core pieces 10 separate from each other.